Superadditive effects of multiple lesions in a connectionist architecture: Implications for the neuropsychology of optic aphasia

Traditional accounts of neuropsychological disorders have been based on the assumption that brain damage results in a single focal lesion of the cognitive architecture. Although this strategy has been productive for neuropsychology, some syndromes defy explanation in this way, resulting in an unparsimonious architecture with many specialized components or connections. Optic aphasia is one such syndrome. Patients with optic aphasia have difficulty naming visually presented objects. However, the deficit is not in visual recognition per se because patients can pantomime the appropriate use of objects, and the deficit is not in naming per se because they can name objects from auditory cues. Rather than supposing that optic aphasia results from damage to a particular pathway in the brain, Farah (1990) conjectured that optic aphasia might arise due to partial damage to two pathways--one that maps visual input to semantics, and the other that maps semantics to naming responses--and the effect of this damage is superadditive, meaning that tasks requiring one pathway or the other show little or no performance deficit, but the damage is manifested when a task requires both pathways (i.e., naming a visually presented object). We have tested this hypothesis by modeling superadditive effects of damage in a connectionist architecture. The resulting model also explained other phenomena associated with optic aphasia, including the tendencies of patients to: produce a large number of naming errors that are semantically related to the target but few visually related errors, show response perseveration from one trial to the next, "home in" on the correct response over time, and make fewer errors on naming from a verbal description than on gesturing the use of an object from a visual presentation. More broadly, superadditive effects of damage provide a novel class of explanations for neuropsychological deficits that might previously have seemed to imply the existence of highly specialized processing components.

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